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11
The Nervous System
The Brain, Cranial Nerves, Autonomic
Nervous System, and the Special Senses
CHAPTER OUTLINE
The principal parts of the brain
After studying this chapter, you should be able to:
The anatomy and function of the brainstem
1. List the principal parts of the brain.
The anatomy and function of the
diencephalon
2. Name the functions of the cerebrospinal fluid.
The cerebrum: structure and function
3. List the principle functions of the major parts of
the brain.
The cerebellum: structure and function
4. List the 12 cranial nerves and their functions.
The autonomic nervous system
5. Name the parts of the autonomic nervous system and describe how it functions.
The 12 cranial nerves and their functions
The special senses: The sense of smell;
The sense of taste; The sense of sight, The
anatomy of the eye; The sense of hearing and
equilibrium
Body systems working together to maintain
homeostasis: the nervous system
230
CHAPTER OBJECTIVES
6. Describe the basic anatomy of the sense organs
and explain how they function.
Common Disease, Disorder or Condition Boxes
Alzheimer’s disease . . . . . . . . . .
Cerebrovascular accident (CVA)
Meningitis . . . . . . . . . . . . . . . . .
Encephalitis . . . . . . . . . . . . . . . .
Tetanus . . . . . . . . . . . . . . . . . . .
Parkinson’s disease . . . . . . . . . .
Cerebral palsy . . . . . . . . . . . . . .
Epilepsy . . . . . . . . . . . . . . . . . .
Headache . . . . . . . . . . . . . . . . .
Otitis media . . . . . . . . . . . . . . . .
Motion sickness . . . . . . . . . . . . .
Conjunctivitis . . . . . . . . . . . . . . .
Myopia . . . . . . . . . . . . . . . . . . .
Hyperopia . . . . . . . . . . . . . . . . .
Presbyopia . . . . . . . . . . . . . . . .
Color blindness . . . . . . . . . . . . .
Glaucoma . . . . . . . . . . . . . . . . .
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KEY TERMS
Abducens nerve VI . . . . . . .238
Parietal lobe . . . . . . . . . . . .235
Accessory nerve XI . . . . . .239
Glossopharyngeal
nerve IX . . . . . . . . . . . . . .238
Aqueous humor . . . . . . . . .243
Gyri . . . . . . . . . . . . . . . . . . .235
Pituitary gland . . . . . . . . . .235
Auditory or
eustachian tube . . . . . . . .245
Hypoglossal nerve XII . . . .239
Pons Varolii . . . . . . . . . . . .235
Hypothalamus . . . . . . . . . . .235
Pupil . . . . . . . . . . . . . . . . . .243
Auricle . . . . . . . . . . . . . . . . .244
Incus . . . . . . . . . . . . . . . . . .245
Reticular formation . . . . . . .234
Autonomic nervous
system . . . . . . . . . . . . . . .236
Infundibulum . . . . . . . . . . .235
Retina . . . . . . . . . . . . . . . . .243
Insula . . . . . . . . . . . . . . . . .236
Rhodopsin . . . . . . . . . . . . .244
Brainstem . . . . . . . . . . . . . .233
Interventricular foramen/foramen of Monroe . . . . . . . .233
Round window . . . . . . . . . .245
Cerebellum . . . . . . . . . . . . .236
Pineal gland . . . . . . . . . . . .235
Sclera . . . . . . . . . . . . . . . . .243
Cerebral aqueduct/aqueduct
of Sylvius . . . . . . . . . . . . .233
Iris . . . . . . . . . . . . . . . . . . . .243
Cerebral cortex . . . . . . . . . .235
Longitudinal fissure . . . . . .235
Cerebral hemispheres . . . .235
Malleus . . . . . . . . . . . . . . . .245
Cerebrum . . . . . . . . . . . . . .235
Mamillary bodies . . . . . . . .235
Cerumen . . . . . . . . . . . . . . .244
Medulla oblongata . . . . . . .233
Ceruminous glands . . . . . .244
Midbrain/mesencephalon . .235
Choroid . . . . . . . . . . . . . . . .243
Occipital lobe . . . . . . . . . . .236
Ciliary body . . . . . . . . . . . .243
Oculomotor nerve III . . . . . .238
Cornea . . . . . . . . . . . . . . . .243
Olfactory nerve I . . . . . . . . .238
Corpus callosum . . . . . . . .235
Olfactory sense . . . . . . . . .240
Decussation of pyramids . .234
Optic chiasma . . . . . . . . . . .235
Diencephalon . . . . . . . . . . .235
Optic disk . . . . . . . . . . . . . .244
Ventral cerebral
peduncles . . . . . . . . . . . .235
Dorsal tectum . . . . . . . . . . .235
Optic nerve II . . . . . . . . . . .238
Ventricles . . . . . . . . . . . . . .233
External auditory meatus . .244
Optic tracts . . . . . . . . . . . . .235
Facial nerve VII . . . . . . . . . .238
Oval window . . . . . . . . . . . .245
Vestibulocochlear
nerve VIII . . . . . . . . . . . . .238
Fovea centralis . . . . . . . . . .244
Papillae . . . . . . . . . . . . . . . .241
Vitreous humor . . . . . . . . . .243
Frontal lobe . . . . . . . . . . . .235
Parasympathetic division . .237
Lens . . . . . . . . . . . . . . . . . .243
Stapes . . . . . . . . . . . . . . . . .245
Sulci . . . . . . . . . . . . . . . . . .235
Sympathetic division . . . . .237
Taste buds . . . . . . . . . . . . .241
Taste cells . . . . . . . . . . . . . .241
Temporal lobe . . . . . . . . . . .236
Thalamus . . . . . . . . . . . . . .235
Trigeminal nerve V . . . . . . .238
Trochlear nerve IV . . . . . . .238
Tympanic membrane . . . . .244
Vagus nerve X . . . . . . . . . . .239
231
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THE PRINCIPAL PARTS OF THE BRAIN
The brain is one of the largest organs of the body
(Figure 11-1). It weighs about 3 pounds in an average adult. It is divided into four major parts: (1) the
brainstem, which consists of three smaller areas,
(the medulla oblongata (meh-DULL-ah ob-longGAH-tah), the pons Varolii (PONZ vah-ROH-leeeye), and the midbrain); (2) the diencephalon,
(dye-en-SEFF-ah-lon) consisting of the thalamus
(THAL-ah-muss) and the hypothalamus; (3) the
cerebrum (seh-REE-brum); and (4) the cerebellum
(seh-ree-BELL-um).
The brain is protected by the cranial bones and
the meninges. The cranial meninges is the name
given to the meninges that protect the brain and
they have the same structure as the spinal
meninges: the outer dura mater, the middle arachnoid mater, and the inner pia mater (discussed in
Chapter 10). The brain, like the spinal cord, is fur-
Cerebrum
Thalamus
Hypothalamus
Pons
Cerebellum
Medulla oblongata
Spinal cord
Parietal lobe
Frontal lobe
Occipital lobe
Lateral sulcus
Temporal lobe
Cerebellum
Medulla
oblongata
FIGURE 11-1. (A) The principal parts of the brain. (B) Photograph of an adult human brain, lateral view.
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ther protected by the cerebrospinal fluid that circulates through the subarachnoid space around the
brain and spinal cord and through the ventricles of
the brain. The ventricles are cavities within the
brain that connect with each other, with the subarachnoid space of the meninges, and with the central canal of the spinal cord. The cerebrospinal fluid
serves as a shock absorber for the central nervous
system and circulates nutrients.
The brain has four ventricles (Figure 11-2). There
are two lateral ventricles in each side or hemisphere
of the cerebrum under the corpus callosum
(KOR-pus kah-LOH-sum). The third ventricle is a slit
between and inferior to the right and left halves of the
thalamus, and situated between the lateral ventricles.
Each lateral ventricle connects with the third ventricle
by a narrow oval opening called the interventricular
foramen or foramen of Monroe. The fourth ventricle lies between the cerebellum and the lower brainstem. It connects with the third ventricle via the cere-
bral aqueduct also known as the aqueduct of
Sylvius. The roof of this fourth ventricle has three
openings through which it connects with the subarachnoid space of the brain and spinal meninges,
thus allowing a flow of cerebrospinal fluid through the
spinal cord, the brain, and the ventricles of the brain.
THE ANATOMY AND
FUNCTIONS OF THE BRAINSTEM
The brainstem consists of the medulla oblongata,
the pons Varolii, and the midbrain. It connects the
brain to the spinal cord. It is a very delicate area of
the brain because damage to even small areas could
result in death. Figure 11-3 shows the parts of the
brain and areas of brain function.
The medulla oblongata contains all the ascending and descending tracts that connect between the
spinal cord and various parts of the brain. These
Third ventricle
Pia mater
Corpus callosum
Subarachnoid
space
Lateral ventricle
Foramen of Monroe
Arachnoid
Cerebrum
Subdural
space
Dura mater
Cerebral aqueduct
Cerebellum
233
Skull
Fourth ventricle
FIGURE 11-2. The ventricles of the brain, the cranial meninges, and the flow pattern of the cerebrospinal fluid.
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Sulci
Convolutions of
cerebral hemisphere
(gyri)
Parietal lobe
Cerebrum
Frontal lobe
Occipital lobe
Temporal lobe
(A)
Midbrain
Brainstem
Pons
Cerebellum
Medulla
Emotions
Personality
Morality
Intellect
Speech
Spe
Lateral View
h
ec
Sensory
Motor
Pain
Heat
Touch
Hearing
Vision
(B)
Smelling
Relays impulses
Autonomic nervous control
Control blood pressure
Maintain body temperature
Stimulates antidiuretic hormone
Assists with appetite regulation
Acts on intestines
Role in emotions
Helps maintain wakefulness
Eye reflexes
Conduct impulses
Breathing
Chewing
Taste
Muscle tone
Equilibrium
Walking
Dancing
Heart
Lungs
Stomach
Blood vessels
FIGURE 11-3. (A) The parts of the brain. (B) Areas of brain function.
tracts make up the white matter of the medulla.
Some motor tracts cross as they pass through the
medulla. The crossing of the tracts is called decussation of pyramids and explains why motor areas
on one side of the cortex of the cerebrum control
skeletal muscle movements on the opposite side of
the body. The medulla also contains an area of dispersed gray matter containing some white fibers.
This area is called the reticular formation, which
functions in maintaining consciousness and arousal.
Within the medulla are three vital reflex centers of
this reticular system: the vasomotor center, which
regulates the diameter of blood vessels; the cardiac
center, which regulates the force of contraction and
heartbeat; and the medullary rhythmicity area,
which adjusts your basic rhythm of breathing.
The pons Varolii is a bridge (pons is Latin for
bridge) that connects the spinal cord with the brain
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and parts of the brain with each other. Longitudinal
fibers connect with the spinal cord or medulla with
the upper parts of the brain, and transverse fibers
connect with the cerebellum. Its pneumotaxic and
apneustic area help control breathing.
The midbrain, also called the mesencephalon
(mess-in-SEFF-ah-lon), contains the ventral cerebral peduncles (seh-REE-bral peh-DUN-kullz) that
convey impulses from the cerebral cortex to the
pons and spinal cord. It also contains the dorsal
tectum, which is a reflex center that controls the
movement of the eyeballs and head in response to
visual stimuli; it also controls the movement of the
head and trunk in response to auditory stimuli, such
as loud noises.
THE ANATOMY AND
FUNCTIONS OF THE DIENCEPHALON
The diencephalon is superior to the midbrain and
between the two cerebral hemispheres. It also surrounds the third ventricle. It is divided into two
main areas: the thalamus and the hypothalamus.
It also contains the optic tracts and optic chiasma
where optic nerves cross each other; the
infundibulum, which attaches to the pituitary
gland; the mamillary bodies, which are involved
in memory and emotional responses to odor; and
the pineal (PIN-ee-al) gland, which is part of the
epithalamus. The pineal gland is a pinecone-shaped
endocrine gland that secretes melatonin, which
affects our moods and behavior. This will be discussed further in Chapter 12.
The thalamus is the superior part of the diencephalon and the principal relay station for sensory
impulses that reach the cerebral cortex coming from
the spinal cord, brainstem, and parts of the cerebrum. It also plays an important role as an interpretation center for conscious recognition of pain
and temperature and for some awareness of crude
pressure and touch.
The epithalamus is a small area superior and posterior to the thalamus. It contains some small nuclei
that are concerned with emotional and visceral
responses to odor. It contains the pineal gland.
The hypothalamus is the inferior part of the
diencephalon and despite its small size it controls
235
many bodily functions related to homeostasis. It
controls and integrates the autonomic nervous system. It receives sensory impulses from the internal
organs. It is the intermediary between the nervous
system and the endocrine system because it sends
signals and controls the pituitary gland, the socalled master gland of the endocrine system. It is
the center for mind-over-body phenomena. When
we hear of unexplainable cures in people diagnosed with terminal illness but who refused to
accept the diagnosis and recovered, the hypothalamus may have been involved in this mind controlling the body phenomenon. It is the hypothalamus
that controls our feelings of rage and aggression. It
controls our normal body temperature. It contains
our thirst center, informing us of when and how
much water we need to sustain our bodies. It maintains our waking state and sleep patterns allowing
us to adjust to different work shifts or jet-lag travel
problems within a day or so. It also regulates our
food intake.
THE CEREBRUM:
STRUCTURE AND FUNCTION
The cerebrum makes up the bulk of the brain. Its
surface is composed of gray matter and is referred
to as the cerebral cortex. Beneath the cortex lies
the cerebral white matter. A prominent fissure, the
longitudinal fissure, separates the cerebrum into
right and left halves or cerebral hemispheres. On
the surface of each hemisphere are numerous folds
called gyri (JYE-rye) with intervening grooves
called sulci (SULL-sigh). The folds increase the surface area of the cortex, which has motor areas for
controlling muscular movements, sensory areas for
interpreting sensory impulses, and association areas
concerned with emotional and intellectual
processes. A deep bridge of nerve fiber known as
the corpus callosum connects the two cerebral
hemispheres (Figure 11-4).
The lobes of the cerebral hemispheres are named
after the bones of the skull that lie on top of them.
The frontal lobe forms the anterior portion of each
hemisphere. It controls voluntary muscular functions, moods, aggression, smell reception, and
motivation. The parietal lobe is behind the frontal
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Frontal lobe
Cerebral cortex
(gray matter)
Gyrus
Sulcus
Superior
Gray
matter
Longitudinal fissure
Central sulcus
White
matter
Corpus callosum
Parietal lobe
Insula
Caudate
nucleus
Lateral sulcus
Putamen
Occipital
lobe
Lateral
ventricle
Globus
pallidus
Lentiform
nucleus
Basal
nuclei
Third ventricle
Inferior
Thalamus
White matter of cerebrum
FIGURE 11-4. The anatomy of the right and left cerebral hemispheres (frontal section).
lobe and is separated from it by the central sulcus.
It is the control center for evaluating sensory information of touch, pain, balance, taste, and temperature. The temporal lobe is beneath the frontal and
parietal lobes and is separated from them by the lateral fissure. It evaluates hearing input and smell as
well as being involved with memory processes. It
also functions as an important center for abstract
thoughts and judgment decisions. The occipital
lobe forms the back portion of each hemisphere; its
boundaries are not distinct from the other lobes. It
functions in receiving and interpreting visual input
(see Figures 11-1 and 11-3). A fifth lobe, the insula,
is embedded deep in the lateral sulcus. The central
sulcus separates the frontal and parietal lobes. The
lateral sulcus separates the cerebrum into frontal,
parietal, and temporal lobes.
THE CEREBELLUM:
STRUCTURE AND FUNCTION
The cerebellum is the second largest portion of the
brain. It is shaped somewhat like a butterfly. It is
located beneath the occipital lobes of the cerebrum
and behind the pons and the medulla oblongata of
the brainstem. It consists of two partially separated
hemispheres connected by a centrally constricted
structure called the vermis. The cerebellum is made
up primarily of white matter with a thin layer of
gray matter on its surface called the cerebellar cortex. It functions as a reflex center in coordinating
complex skeletal muscular movements, maintaining
proper body posture, and keeping the body
balanced. If damaged, there can be a decrease in
muscle tone, tremors, a loss of equilibrium, and difficulty in skeletal muscle movements.
THE AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system is a subdivision of
the efferent peripheral nervous system. It functions
automatically without conscious effort. It regulates
the functions of internal organs by controlling
glands, smooth muscles, and cardiac muscle. It
assists in maintaining homeostasis by regulating
heartbeat and blood pressure, breathing, and body
temperature. This system helps us to deal with emergency situations, emotions, and physical activities.
Receptors within organs send sensory impulses
to the brain and spinal cord. Motor impulses travel
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The Brain, Cranial Nerves, Autonomic Nervous
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along peripheral nerve fibers that lead to ganglia
outside the central nervous system within cranial
and spinal nerves. These ganglia are part of the
autonomic nervous system.
There are two parts to the autonomic nervous
system. The sympathetic division (Figure 11-5)
prepares the body for stressful situations that require
energy expenditure, such as increasing heartbeat
and breathing rates to flee from a threatening situation. The fibers of the system arise from the thoracic
and lumbar regions of the spinal cord. Their axons
leave the cord through the ventral roots of the spinal
nerves but then leave the spinal nerve and enter
members of a chain of paravertebral ganglia extending longitudinally along the side of the vertebral
column. Leaving the paravertebral ganglion, another
neuron, the postganglionic fiber, goes to the effector
organ. The sympathetic division uses acetylcholine
in the preganglionic synapses as a neurotransmitter
but uses norepinephrine (or noradrenalin) at the
synapses of the postganglionic fibers.
The parasympathetic division operates under
normal nonstressful conditions. It also functions in
restoring the body to a restful state after a stressful
experience, thus counterbalancing the effects of the
sympathetic division. The preganglionic fibers of
the parasympathetic division arise from the brainstem and the sacral region of the spinal cord (Figure
11-6). They lead outward in the cranial and sacral
nerves to ganglia located close to the viscera. The
postganglionic fibers are short and go to the muscles or glands within the viscera to bring about their
effects. The preganglionic and the postganglionic
fibers of the parasympathetic division use acetylcholine as the neurotransmitter into the synapses.
Most organs that receive autonomic motor neurons are innervated by both the parasympathetic
and sympathetic divisions. However, there are some
exceptions: blood vessels and sweat glands are
innervated by sympathetic neurons, and smooth
muscles associated with the lens of the eye are controlled by parasympathetic neurons.
The sympathetic division prepares us for physical activity by increasing blood pressure and heartbeat rate, it dilates respiratory passageways for
increased breathing rates, and stimulates sweating.
It also causes the release of glucose from the liver
as a quick source of energy while inhibiting diges-
Lacrimal gland
and nasal septum
Midbrain
Medulla
Paravertebral
chain
ganglion
Eye
Parotid
gland
Submandibular
and sublingual
salivary glands
Trachea
T1
T2
Heart
T3
Lung
T4
T5
Celiac ganglion
Stomach
T6
T7
Pancreas
T8
Small intestine
T9
T10
Liver
Spleen
T11
Adrenal gland (medulla)
T12
L1
L2
Superior
mesenteric
ganglion
Large intestine
Kidney
Inferior
mesenteric
ganglion
Urinary bladder
and genitals
FIGURE 11-5. The nerve pathways of the sympathetic division of the
autonomic nervous system.
tive activities. This system is occasionally called the
fight or flight system because it prepares us to face
a threat or flee quickly from it.
The parasympathetic division stimulates digestion, urination, and defecation. It also counteracts
the effects of the sympathetic division by slowing
down heartbeat rate, lowering blood pressure, and
slowing the breathing rate. It is also responsible
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Ciliary ganglion
Cranial nerve III
Midbrain
Cranial
nerve VII
for the constriction of the pupil of the eye. This
division is occasionally called the rest and repose
system.
Medulla
THE 12 CRANIAL NERVES AND
THEIR FUNCTIONS
Pterygopalatine
ganglion
Submandibular
ganglion
Cranial
nerve IX
T1
Otic ganglion
T2
Cranial nerve X
T3
T4
Lung
T5
Heart
T6
T7
Liver
T8
T9
Stomach
T10
T11
Spleen
T12
Pancreas
Small intestine
L1
L2
Large intestine
Kidney
S2
Urinary
bladder and
genitals
S3
Pelvic nerves
S4
FIGURE 11-6. The nerve pathways of the parasympathetic division of
the autonomic nervous system.
There are 12 pairs of cranial nerves. Ten pairs originate from the brainstem. All 12 pairs leave the skull
through various foramina of the skull. They are designated in two ways: by Roman numerals indicating
the order in which the nerves arise from the brain
(from the front of the brain to the back) and by
names that indicate their function or distribution.
Some cranial nerves are only sensory or afferent,
others are only motor or efferent. Cranial nerves
with both sensory and motor functions are called
mixed nerves (Figure 11-7).
The olfactory nerve (I) is entirely sensory and
conveys impulses related to smell. The
optic nerve (II) is also entirely sensory and conveys impulses related to sight. The oculomotor
nerve (III) is both a sensory and a motor or mixed
nerve. It controls movements of the eyeball and
upper eyelid and conveys impulses related to muscle sense or position called proprioception. Its
parasympathetic function causes constriction of the
pupil of the eye. The trochlear nerve (IV) is a
mixed nerve. It controls movement of the eyeball
and conveys impulses related to muscle sense. It is
the smallest of the cranial nerves. The trigeminal
nerve (V) is also a mixed nerve and it is the largest
of the cranial nerves. It has three branches: the
maxillary, the mandibular, and the ophthalmic. It
controls chewing movements and delivers impulses
related to touch, pain, and temperature in the teeth
and facial area. The abducens nerve (VI) is a
mixed nerve that controls movement of the eyeball.
The facial nerve (VII) is a mixed nerve. It controls the muscles of facial expression and conveys
sensations related to taste. Its parasympathetic
function controls the tear and salivary glands. The
vestibulocochlear nerve (VIII) (ves-tib-yooloh-KOK-lee-ar NERV) is entirely sensory. It
transmits impulses related to equilibrium and
hearing. The glossopharyngeal nerve (IX)
(GLOSS-oh-fair-in-GEE-al NERV) is a mixed
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239
Olfactory nerve (I)
Optic nerve (II)
Ophthalmic branch
Oculomotor nerve (III)
Maxillary branch
Mandibular branch
Trochlear nerve (IV)
Trigeminal nerve
(V)
Abducens nerve (VI)
Glossopharyngeal (IX)
and vagus (X) nerves
Facial (VII) and
vestibulocochlear
(VIII) nerves
Accessory nerve (XI)
Hypoglossal nerve
(XII)
FIGURE 11-7. The cranial nerves are named by Roman numerals or by name indicating distribution or function.
nerve. It controls swallowing and senses taste. Its
parasympathetic function controls salivary glands.
The vagus nerve (X) is a mixed nerve. It controls
skeletal muscle movements in the pharynx, larynx, and palate. It conveys impulses for sensations in the larynx, viscera, and ear. Its parasympathetic function controls viscera in the thorax
and abdomen. The accessory nerve (XI) is a
mixed nerve. It originates from the brainstem and
the spinal cord. It helps control swallowing and
movements of the head. It senses impulses of
muscle position or sense. Finally, the hypoglossal nerve (XII) is also a mixed nerve. It controls
the muscles involved in speech and swallowing
and its sensory fibers conduct impulses for mus-
cle sense. Table 11-1 provides a summary of the
names and functions of the cranial nerves.
THE SPECIAL SENSES
The five special senses are smell, taste, vision, hearing, and balance. The senses of smell and taste are
initiated by the interactions of chemicals with sensory receptors on the tongue and in the nose.
Vision occurs due to the interaction of light with
sensory receptors in the eye. Hearing and balance
function due to the interaction of mechanical stimuli (sound waves for hearing and motion for balance) with sensory receptors in the ear.
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The Cranial Nerves
Number
Name
Function
I
Olfactory
Sensory: smell
II
Optic
Sensory: vision
III
Oculomotor
Motor: movement of the eyeball, regulation of the size of the pupil
IV
Trochlear
Motor: eye movements
V
Trigeminal
Sensory: sensations of head and face, muscle sense
Motor: mastication
Note: divided into three branches: the ophthalmic branch, the maxillary branch, and the mandibular branch
VI
Abducens
Motor: movement of the eyeball, particularly abduction
VII
Facial
Sensory: taste
Motor: facial expressions, secretions of saliva
VIII
Vestibulocochlear
Sensory: balance, hearing
Note: divided into two branches: the vestibular branch responsible
for balance and the cochlear branch responsible for hearing
IX
Glossopharyngeal
Sensory: taste
Motor: swallowing, secretion of saliva
X
Vagus
Sensory: sensation of organs supplied
Motor: movement of organs supplied
Note: supplies the head, pharynx, bronchus, esophagus, liver, and
stomach
XI
Accessory
Motor: shoulder movement, turning of head, voice production
XII
Hypoglossal
Motor: tongue movements
The Sense of Smell
The sense of smell is also known as the olfactory
(ol-FAK-toh-ree) sense. Molecules in the air enter
the nasal cavity and become dissolved in the
mucous epithelial lining of the superior nasal conchae, the uppermost shelf area in the nose (Figure
11-8A). Here they come in contact with olfactory
neurons modified to respond to odors. These neu-
rons are bipolar neurons. Their dendrites are found
in the epithelial surface of the uppermost shelf and
contact the olfactory receptor sites in the nose. The
odor molecules bind to these receptor sites. The
olfactory neurons transmit the impulse along their
axons whose ends become enlarged olfactory
bulbs. From here, they connect with association
neurons to the area of the brain called the olfactory
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Nerve fibers within the olfactory bulb
Olfactory tract
Olfactory bulb
Olfactory area of
nasal cavity
Cribriform plate
Columnar
epithelial cells
Superior nasal
concha
Nasal cavity
Olfactory
receptor cells
Cilia
(A)
(B)
FIGURE 11-8. (A) The olfactory area in the nose formed by the superior nasal conchae. (B) Columnar epithelial cells support olfactory receptor
cells with cilia at their ends.
cortex found in the temporal and frontal lobes of
the cerebrum.
The receptor cells are neurons that have cilia at
the distal ends of their dendrites (Figure 11-8B). It
is these cilia that function as chemoreceptors to
detect odors. These molecules first become dissolved in the mucous membrane that lines the
olfactory shelf in the nose and then are detected.
The sense of smell is closely related to the sense of
taste. We use these two senses to decide whether or
not to eat a particular food. Our sense of smell is
complex because a small number of receptors
detect a great variety of odors. It is the brain that
then interprets these receptor combinations into a
type of olfactory code. The exact mechanism of
how this works is still being investigated by biologists. However, we do know that olfactory receptors rapidly adapt to odors and after a short time we
no longer perceive the odor as intensely as it was
initally detected.
The Sense of Taste
Taste buds are the sensory structures found on certain papillae (pah-PILL-ay), which are elevations of
the tongue, that detect taste stimuli (Figure 11-9).
Taste buds are also found on the palate of the roof
of the mouth and in certain regions of the pharynx.
Each taste bud is composed of two types of cells.
The first type are specialized epithelial cells that form
the exterior capsule of the taste bud. The second
type of cell forms the interior of the taste bud. These
cells are called taste cells and function as the receptor sites for taste. The taste bud is spherical with an
opening called the taste pore. Taste hairs are tiny
projections of the taste cells that extend out of the
taste pore. It is these taste hairs that actually function
as the receptors of the taste cell. Cranial nerves VIII,
IX, and X conduct the taste sensations to the brain,
which perceives and interprets the taste.
Before a chemical can be tasted, it must first be
dissolved in a fluid (just like the odors in the nose).
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Papillae
Taste cell
Taste pore
(A)
Taste
buds
Supporting
cell
Sensory
nerve
fiber
Taste hair
Connective
tissue
Epithelium
of tongue
(B)
(C)
(D)
(E)
(F)
FIGURE 11-9. (A) Taste buds on the surface of the tongue are associated with elevations called papillae. (B) A taste bud contains taste cells
with an opening called the taste pore at its free surface. Colored sections indicate common patterns of taste receptors: (C) Sweet (D) Sour
(E) Salt (F) Bitter
The saliva produced by the salivary glands provides
this fluid medium. Nerve fibers surrounding the
taste cells transmit the impulses to the brain for
interpretation. The sensory impulses travel on the
facial (VIII), glossopharyngeal (IX), and vagus (X)
cranial nerves to the gustatory (taste) cortex of the
parietal lobe of the cerebrum for interpretation. The
four types of taste sensations are sweet, sour, salty,
and bitter. Although all taste buds can detect all four
sensations, taste buds at the back of the tongue
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react strongly to bitter, taste buds at the tip of the
tongue react strongly to sweet and salty, and taste
buds on the side of the tongue respond more
strongly to sour tastes (see Figure 11-9). Taste sensations are also influenced by olfactory sensations.
Holding one’s nose while swallowing reduces the
taste sensation. This is a common practice when
taking bad tasting medicine.
The Sense of Sight
The eyes are our organs of sight. They are protected
by the orbits of the skull. See Chapter 7 to review
the bones that make up the orbits. In addition, the
eyebrows help shade the eye and keep perspiration
from getting into the eye and causing an irritation
to the eye. Eyelids and eyelashes protect the eye
from foreign objects. Blinking of the eyelids lubricates the surface of the eye by spreading tears that
are produced by the lacrimal gland. The tears not
only lubricate the eye but also help to combat bacterial infections through the enzyme lysozyme, salt,
and gamma globulin.
The Anatomy of the Eye
The eye is a sphere filled with two fluids (Figure
11-10). The skeletal muscles that move the eye
were discussed in Chapter 9. They are the rectus
muscles and the oblique muscles.
The wall of the eye is composed of three layers,
or tunics of tissue. The outermost layer is the
sclera (SKLAIR-ah). It is white and composed of
tough connective tissue. We see it as the white of
the eye when looking in a mirror. The cornea
(COR-nee-ah) is the transparent part of this outermost layer that permits light to enter the eye. The
second layer is the choroid (KOR-oyd). It contains
numerous blood vessels and pigment cells. It is
black in color and absorbs light so that it does not
reflect in the eye and impair vision. The innermost
layer of the eye is the retina (RET-ih-nah). It is
gray in color and contains the light-sensitive cells
known as the rods and cones.
The ciliary (SIL-ee-air-ee) body consists of
smooth muscles that hold the biconvex, transparent
and flexible lens in place. The iris is the colored
part of the eye consisting of smooth muscle that
surrounds the pupil. The iris regulates the amount
of light that enters through the diameter of the
pupil. When we go into a dark room, the iris opens
to allow more light to enter. When we go out into
strong sunlight, the iris constricts letting less light
enter the pupil.
The interior of the eye is divided into two compartments. In front of the lens is the anterior compartment that is filled with a fluid called the aqueous humor. This fluid helps to bend light, is a
source of nutrients for the inner surface of the eye,
and maintains ocular pressure. It is produced by the
ciliary body. The posterior compartment of the eye
is filled with vitreous (VIT-ree-us) humor. It too
Ciliary body
and muscle
Suspensory
ligament
Conjunctiva
Retina
Iris
Retinal arteries
and veins
Pupil
Path of light
Fovea
centralis
Anterior chamber
(aqueous humor)
Cornea
Optic
nerve
Lens
Posterior chamber
vitreous humor
FIGURE 11-10. The anatomy of the eye, transverse view.
243
Choroid coat
Sclera
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helps to maintain ocular pressure, refracts or bends
light, and holds the retina and lens in place.
The retina is the innermost layer of the eye and
contains the photosensitive cells (Figure 11-11). The
retina has a pigmented epithelial layer that helps
keep light from being reflected back into the eye.
The sensory layer is made up of the rods and cones.
There are more rods than cones in this layer. Rods
are quite sensitive to light and function in dim light,
but do not produce color vision. It is the cones that
produce color and they require lots of light. Three
different types of cones are sensitive to either red,
green, or blue. Combinations of these cones produce all the other colors we see.
The rod and cone cells synapse with the bipolar
cells of the retina. The bipolar cells synapse with
ganglia cells whose axons form the optic nerve.
Eventually the fibers of the optic nerve reach the
thalamus of the brain and synapse at its posterior
portion and enter as optic radiations to the visual
cortex of the occipital lobe of the cerebrum for
interpretation.
The yellowish spot in the center of the retina is
called the macula lutea. In its center is a depression
called the fovea centralis. This region produces
the sharpest vision, like when we look directly at an
object. Medial to the fovea centralis is the optic
disk. It is here that nerve fibers leave the eye as the
Ganglion cell
Secondary
neuron
Rods–
dim light
Retina
Cell
bodies
Cones–
responsible
for colors and
bright lights
Optic nerve (11)
FIGURE 11-11. The layers of the retina illustrating the rods and
cones and other cellular layers.
optic nerve. Because the optic disk has no receptor
cells, it is called the blind spot.
Both rods and cones contain light-sensitive pigments. Rod cells contain the pigment called
rhodopsin (roh-DOP-sin). Cone cells contain a
slightly different pigment. When exposed to light
the rhodopsin breaks down into a protein called
opsin and a pigment called retinal. Manufacture of
retinal requires vitamin A. Someone with a vitamin
A deficiency may experience night blindness, which
is difficulty seeing in dim light.
Sight is one of our most important senses. Humans
depend on sight as their main sense to survive and
interact with our environment. We educate ourselves
via visual input through reading, color interpretations,
and movement. People who lose their sight tend to
develop acuity with the other senses like smell and
sounds, senses that our dog and cat companions have
developed to a high degree.
The Sense of Hearing and Equilibrium
The external, inner, and middle ear contain the
organs of balance and hearing (Figure 11-12). The
external ear is that part of the ear that extends from
the outside of the head to the eardrum. Medial to
the eardrum is the air-filled chamber called the middle ear, which contains the auditory ossicles: the
malleus, incus, and stapes. The external and middle
ear are involved in hearing. The inner ear is a group
of fluid-filled chambers that are involved in both
balance and hearing.
The external ear consists of the flexible, visible
part of our ear called the auricle (AW-rih-kl) composed mainly of elastic cartilage. This connects with
our ear canal known as the external auditory
meatus (eks-TER-nal AW-dih-tor-ee mee-ATE-us).
The auricle allows sound waves to enter the ear
canal, which then directs those waves to the delicate eardrum or tympanic (tim-PAN-ik) membrane. The ear canal is lined with hairs and modified sebaceous glands called ceruminous
(seh-ROO-men-us) glands that produce earwax or
cerumen. The hairs and earwax protect the
eardrum from foreign objects. The thin tympanic
membrane, which is silvery gray in color, is very
delicate and sound waves cause it to vibrate.
The middle ear is the air-filled cavity that contains the three auditory ossicles or ear bones: the
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Middle ear
Incus
Semicircular
canals
Malleus
Branches of
vestibulocochlear
nerve
Auricle
Cochlea
External
auditory canal
Inner
ear
Oval window
Round window
Auditory
(eustachian) tube
Tympanic membrane
Stapes and footplate
FIGURE 11-12. The external, middle, and inner ear and their organs.
malleus or hammer, the incus or anvil, and the
stapes or stirrup. These bones transmit the sound
vibrations from the eardrum to the oval window.
The two openings on the medial side of the middle
ear are the oval window and the round window.
They connect the middle ear to the inner ear. As the
vibrations of the sound waves are transmitted from
the malleus to the stapes, they are amplified in the
middle ear. In the middle ear we also find the auditory or Eustachian (yoo-STAY-shun) tube. This
tube opens into the pharynx and permits air pressure to be equalized between the middle ear and
the outside air, thus ensuring that hearing is not distorted. When flying in an airplane, changing altitude changes pressure. This results in muffled
sounds and pain in the delicate eardrum. We can
allow air to enter or exit the middle ear through the
auditory tube and thus equalize the pressure by
yawning, chewing, or swallowing. Sometimes we
hold our nose and mouth shut and gently force air
out of our lungs through the auditory tube and pop
our eardrum to equalize the pressure.
The inner ear is made of interconnecting chambers and tunnels within the temporal bone. This
area contains the cochlea, which is involved in
hearing, and the vestibule and the semicircular
canals, which are involved in balance. Balance is
also called equilibrium. Static equilibrium is controlled by the vestibule and determines the position
of the head in relation to gravity; kinetic equilibrium is controlled by the semicircular canals and
determines the change in regard to head rotational
movements.
BODY SYSTEMS WORKING TOGETHER
TO MAINTAIN HOMEOSTASIS:
THE NERVOUS SYSTEM
Integumentary System
Temperature receptors in the skin detect
changes in the external environment and
transmit this information to the nervous system
for interpretation about hot and cold sensations.
Pressure receptors in the skin detect changes
in the external environment and transmit this
information to the nervous system for
interpretation about pleasure and pain
sensations.
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Common Disease, Disorder or Condition
DISORDERS OF THE
NERVOUS SYSTEM
Alzheimer’s disease
Alzheimer’s (ALTS-high-mers) disease
results in severe mental deterioration. It is also
known as senile dementia-Alzheimer type
(SDAT). It usually affects older people but may
begin in middle life with symptoms of memory
loss and behavioral changes. The disease
affects 10% of people older than 65 and
nearly half of those 85 or older. Symptoms
worsen dramatically in individuals older than
70. Symptoms include memory failure, confusion, a decrease in intellectual capacity, restlessness, disorientation, and occasionally
speech disturbances. The disease produces a
loss of neurons in the cerebral cortex of the
brain resulting in a decrease in brain size. The
sulci widen and the gyri become narrowed.
The temporal and frontal lobes of the cerebrum are particularly affected. Enlarged axons
containing beta-amyloid protein, called
plaques, form in the cortex. There is a genetic
predisposition for the disease. The first symptoms of the disease usually begin with an
inability to assimilate new information despite
the ability to retain old knowledge, difficulty in
recalling words, and a disorientation in common surroundings. Death usually occurs 8 to
12 years after the onset of symptoms. The
disease has no cure. The patient should be
kept comfortable and carefully observed to
keep the individual from self-harm.
Cerebrovascular Accident (CVA)
Cerebrovascular (seh-REE-bro-VAS-kyoolar) accident (CVA) or stroke can be caused
by a clot or thrombus in a blood vessel, or by
a piece of a clot or embolus that breaks loose
and travels in the circulatory system until it
lodges in a blood vessel and blocks circulation. It can also be caused by a hemorrhage in
tissue or by the constriction of a cerebral
blood vessel, known as a vasospasm. These
situations can result in localized cellular death
due to lack of blood supply to the tissue. This
is known as an infarct. Symptoms are determined by the size and location of the infarct
but can include paralysis or lack of feeling on
the side of the body opposite the cerebral
infarct, weakness, speech defects, or the
inability to speak. Death may result. However,
symptoms may subside in minor strokes when
the resulting brain swelling subsides.
Meningitis
Meningitis (men-in-JYE-tis) is an inflammation of the meninges caused by bacterial
or viral infection, which results in headache,
fever, and a stiff neck. In severe cases it
can result in paralysis, coma, and death.
Encephalitis
Encephalitis (in-seff-ah-LYE-tis) is an
inflammation of brain tissue usually caused
by a virus, transmitted by the bite of a mosquito. It is manifested by a wide variety of
symptoms including coma, fever, and convulsions, and could result in death.
Tetanus
Tetanus is caused by the introduction of
the bacterium Clostridium tetani into an
open wound. The bacterium produces a
neurotoxin that affects motor neurons in the
spinal cord and brainstem. It also blocks
inhibitory neurotransmitters resulting in
muscle contractions. The jaw muscles are
affected earliest locking the jaw in a closed
position, hence the common name lockjaw.
Death can result from spasms of the respiratory muscles and the diaphragm.
Parkinson’s disease
Parkinson’s disease is characterized by
tremors of the hand when resting and a
slow shuffling walk with rigidity of muscular
movements. It is caused by damage to
basal nuclei resulting in deficient dopamine,
(continues)
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Common Disease, Disorder or Condition (continued)
an inhibitory neurotransmitter. The disease
can be treated to a certain degree with Ldopa. New research uses the transplantion
of fetal cells from discarded umbilical cords
into the patient. These cells can produce
dopamine in the individual with the disease.
Cerebral palsy
Cerebral palsy (seh-REE-bral PAWL-zee)
is a condition caused by brain damage during brain development or the birth process.
The child’s motor functions and muscular
coordinations are defective. Symptoms
include awkward movements, head tossing,
and flailing arms. Speaking is impaired with
gutteral sounds and swallowing is difficult.
Body balance is poor, with spasms and
tremors of muscles. Careful prenatal and
Skeletal System
The skull bones and vertebrae protect the
brain and spinal cord.
Bones store calcium for release into the
blood. Calcium is necessary for nervous
transmission.
Muscular System
Muscular contraction depends on nerve
stimulation.
Muscle sense and position of body parts are
controlled by sensory neurons and
interpretations by the nervous system.
Endocrine System
The hypothalamus of the brain, through
neurosecretions, controls the actions of the
pituitary gland, the master gland of the
endocrine system, which controls the
secretions of many hormones of other
endocrine glands.
Cardiovascular System
Nerve impulses control heartbeat and blood
pressure.
Nerve impulse control dilation and constriction
of blood vessels, thus controlling blood flow.
obstetric care is necessary to prevent this
condition.
Epilepsy
Epilepsy is caused by a disorder in the
brain where certain parts of the brain are
overactive, producing convulsive seizures
(involuntary muscle contractions) and possible loss of consciousness.
Headache
Headache or cephalalgia can be caused by
a variety of factors from muscle tension
and anxiety to swollen sinuses and
toothache. Headache can also be caused
by inflammation of the meninges, brain
tumors, and vascular changes in the blood
supply to the brain.
Lymphatic System
Nervous anxiety and stress can impair the
immune response, a major function of the
lymphatic system.
The hypothalamus controls mind over body
phenomena and boosts the immune response,
thus fighting disease.
Digestive System
The autonomic nervous system controls
peristalsis resulting in mixing of food with
digestive enzymes and moving food along the
digestive tract.
Nerve impulses inform us when to empty the
tract of indigestible waste.
Respiratory System
Respiratory rates are controlled by the nervous
system thus controlling oxygen and carbon
dioxide levels in the blood.
The phrenic nerve controls the action of the
diaphragm muscle, which controls breathing
rates.
Urinary System
Nerve impulses to the kidneys control the
composition and concentration of urine.
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DISORDERS OF THE SENSES
Otitis media (oh-TYE-tis MEE-dee-ah) or middle
ear infection is quite common in young children. It
can result in a temporary loss of hearing due to
fluid buildup near the tympanic membrane.
Symptoms include fever and irritability, and on
examination, a red eardrum.
Motion sickness is caused by constant stimulation of the semicircular canals of the inner ear due
to the motion of a car, boat, or airplane, resulting
in nausea and weakness.
Conjunctivitis (kon-junk-tih-VYE-tis) is caused by
a bacterial infection of the conjunctiva of the eye.
Contagious conjunctivitis is called pinkeye and is
common in children. It can be transmitted easily
by hand to eye contact or by contaminated water
in a swimming pool.
Myopia (my-OH-pee-ah) is commonly called nearsightedness. It is the ability to see close objects
but not distant ones.
Stretch receptors in the bladder inform us
when to eliminate urine from the body.
Reproductive System
Sperm and egg production is stimulated by the
nervous system at the beginning of puberty
and throughout life in men and up to
menopause in women.
Sexual pleasure is determined by sensory
receptors in various parts of the body.
Smooth muscle contractions stimulated by the
nervous system initiate childbirth and delivery.
Sucking at the breast by the newborn stimulates
milk production in the mammary glands.
Summary Outline
THE PRINCIPAL PARTS OF THE BRAIN
1. The brain is divided into four main parts: the
brainstem consisting of the medulla oblongata,
Hyperopia (high-per-OH-pee-ah) is commonly
called farsightedness. It is the ability to see distant
objects but not close ones. Both myopia and
hyperopia can be corrected by a corrective lens (a
concave lens for myopia and a convex lens for
hyperopia).
Presbyopia (prez-bee-OH-pee-ah) is a decrease
in the ability of the eye to accommodate for near
vision. This is a normal part of aging and commonly occurs during the forties. It can be corrected by the use of reading glasses.
Color blindness is an X chromosome inherited
genetic trait occurring more frequently in males. It
results in the inability to perceive one or more
colors.
Glaucoma (glah-KOH-mah) is caused by excessive pressure buildup in the aqueous humor,
which can constrict blood vessels entering the
eye. This causes destruction of the retina or optic
nerve resulting in blindness.
the pons Varolii, and the midbrain; the
diencephalon consisting of the thalamus and
the hypothalamus; the cerebrum consisting of
two hemispheres; and the cerebellum.
2. The brain is protected by the cranial bones,
the cranial meninges, and the cerebrospinal
fluid.
3. Cerebrospinal fluid acts as a shock absorber
for the central nervous system and circulates
nutrients. In the brain it circulates in the
subarachnoid space and the four ventricles.
THE ANATOMY AND FUNCTION OF THE BRAINSTEM
1. The medulla oblongata contains all the
ascending and descending tracts that connect
the spinal cord with the brain. Some of these
tracts cross in the medulla, known as
decussation of pyramids. This explains why
motor functions on one side of the cerebrum
control muscular movements on the opposite
side of the body.
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2. The reticular formation of the medulla
controls consciousness and arousal. The
three vital reflex centers control the
diameter of blood vessels, heartbeat, and
breathing rates.
3. The pons Varolii is a bridge that connects the
spinal cord with the brain and parts of the
brain with each other. It also helps control
breathing.
4. The midbrain or mesencephalon contains the
dorsal tectum, a reflex center, that controls
movement of the head and eyeballs in
response to visual stimulation and movement
of the head and trunk in response to auditory
stimuli.
THE ANATOMY AND FUNCTIONS OF THE DIENCEPHALON
1. The thalamus is a relay station for sensory
impulses and an interpretation center for
recognition of pain, temperature, and crude
touch.
2. The hypothalamus controls functions related
to homeostasis: it controls the autonomic
nervous system; it receives sensory impulses
from the viscera; it controls the pituitary
gland; it is the center for mind-over-body
phenomena; it controls our thirst center;
and it maintains our waking and sleep
patterns.
THE CEREBRUM: STRUCTURE AND FUNCTION
1. The surface of the cerebrum is composed of
gray matter and is called the cerebral cortex.
Below the cortex is the white matter.
2. A longitudinal fissure separates the cerebrum
into two hemispheres. Folds on the surface of
the hemispheres are called gyri with
intervening grooves called sulci.
3. The corpus callosum is a bridge of nerve
fibers that connect the two hemispheres.
4. The surface of the cortex has motor areas to
control muscular movements, sensory areas for
interpreting sensory impulses, and association
areas concerned with emotional and
intellectual processes.
5. Each hemisphere is divided into four main
lobes.
6. The frontal lobe controls voluntary muscular
movements, moods, aggression, smell
reception, and motivation.
249
7. The parietal lobe evaluates sensory
information concerning touch, pain, balance,
taste, and temperature.
8. The temporal lobe evaluates hearing, smell,
and memory. It is a center for abstract thought
and judgment decisions.
9. The occipital lobe evaluates visual input.
THE CEREBELLUM: STRUCTURE AND FUNCTION
1. The cerebellum consists of two partially
separated hemispheres connected by a
structure called the vermis. The cerebellum is
shaped like a butterfly.
2. It functions as a center for coordinating
complex muscular movements, maintaining
body posture, and balance.
THE AUTONOMIC NERVOUS SYSTEM
1. The autonomic nervous system is a
subdivision of the efferent peripheral nervous
system.
2. It regulates internal organs by controlling
glands, smooth muscle, and cardiac muscle. It
maintains homeostasis by regulating heartbeat,
blood pressure, breathing, and body
temperature.
3. It helps us to control emergency situations,
emotions, and various physical activities.
4. It consists of two subdivisions: the sympathetic
division and the parasympathetic division.
5. The sympathetic division deals with energy
expenditure and stressful situations by
increasing heartbeat rates and breathing. Its
fibers arise from the thoracic and lumbar
regions of the spinal cord. It uses
acetylcholine as a neurotransmitter in the
preganglionic synapses and norepinephrine or
noradrenalin at postganglionic synapses.
6. The parasympathetic division functions in
restoring the body to a nonstressful state. Its
fibers arise from the brainstem and the sacral
region of the spinal cord. It uses acetylcholine
at both the preganglionic and postganglionic
synapses as a neurotransmitter.
7. The sympathetic division prepares us for
physical activity: it increases blood pressure,
heart rate, breathing, and sweating; it
releases glucose from the liver for quick
energy. It is also known as the fight or
flight system.
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8. The parasympathetic division counteracts the
effects of the sympathetic division: it slows
down heart rate, lowers blood pressure and
slows breathing. It also controls digestion,
urination, defecation, and constriction of the
pupil. It is known as the rest or repose system.
THE 12 CRANIAL NERVES AND THEIR FUNCTIONS
1. Olfactory nerve (I) conveys impulses related to
smell. It is sensory.
2. Optic nerve (II) conveys impulses related to
sight. It is sensory.
3. Oculomotor nerve (III) controls movements of
the eyeballs and upper eyelid. Its
parasympathetic function controls constriction
of the pupil. It is both sensory and motor.
4. Trochlear nerve (IV) controls movement of the
eyeball. It is both sensory and motor.
5. Trigeminal nerve (V) controls chewing
movements and senses touch, temperature and
pain in the teeth and facial area. It is both
sensory and motor.
6. Abducens nerve (VI) also controls movement
of the eyeball. It is both sensory and motor.
7. Facial nerve (VII) controls the muscles of
facial expression. It also senses taste. Its
parasympathetic function controls the tear and
salivary glands. It is both sensory and motor.
8. Vestibulocochlear nerve (VIII) transmits
impulses related to equilibrium and hearing. It
is sensory.
9. Glossopharyngeal nerve (IX) controls
swallowing and senses taste. Its
parasympathetic function controls salivary
glands. It is both sensory and motor.
10. Vagus nerve (X) controls skeletal muscle
movements in the pharynx, larynx, and palate.
It conveys sensory impulses in the larynx,
viscera, and ear. Its parasympathetic functions
control viscera in the thorax and abdomen. It
is both sensory and motor.
11. Accessory nerve (XI) helps control swallowing
and movement of the head. It is both sensory
and motor.
12. Hypoglossal nerve (XII) controls muscles
involved in swallowing and speech. It is both
sensory and motor.
THE SPECIAL SENSES
1. The senses of smell and taste are initiated
by the interactions of chemicals with
sensory receptors on the tongue and in the
nose.
2. The sense of vision occurs due to the
interactions of light with sensory receptors in
the eye.
3. The senses of hearing and balance occur due
to the interaction of sound waves for hearing
and motion for balance with sensory receptors
in the ear.
The Sense of Smell
1. The sense of smell, or the olfactory sense,
occurs because molecules in the air become
dissolved in the mucous epithelial lining of the
superior nasal conchae of the nose.
2. Bipolar sensory neurons transfer these
chemical impulses to the olfactory bulbs that
connect with association neurons of the
olfactory cortex in the temporal and frontal
lobes of the cerebrum.
3. A small number of receptors in the nose
detect a great variety of odors via brain
interpretation of receptor combinations.
The Sense of Taste
1. Taste buds are found on certain papillae of the
tongue, on the palate of the roof of the
mouth, and part of the pharynx.
2. Taste buds consist of two types of cells:
epithelial cells that form the exterior capsule
and taste cells that form the interior of the
taste bud.
3. The taste chemical is first dissolved in the fluid
of saliva. These sensory impulses are
conducted by the facial, glossopharyngeal, and
vagus nerves to the taste cortex of the parietal
lobe of the cerebrum for interpretation.
4. There are four types of taste sensations:
bitter, strongly detected at the back of the
tongue; sweet and salty, detected at the tip
of the tongue; and sour, detected more
strongly by the taste buds on the sides of the
tongue.
5. Taste sensations are also influenced by
olfactory sensations.
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The Sense of Sight
1. The eyes are the organs of sight. Eyelids and
eyelashes protect the eyes from foreign
objects. Tears, produced by the lacrimal
glands, lubricate the eyes.
2. Tears contain the bacteriolytic enzyme
lysozyme.
The Anatomy of the Eye
1. The wall of the eye is composed of three
layers: the sclera, the choroid, and the retina.
2. The sclera is the outermost, white, hard layer
composed of tough collagenous connective
tissue.
3. The cornea is the transparent part of the sclera
that allows light to enter the eye.
4. The choroid is the second layer and contains
blood vessels and pigment cells. It is black in
color and absorbs light to prevent reflection
that could impair vision.
5. The retina is the innermost layer of the eye. It
contains the light-sensitive cells called rods
and cones.
6. The ciliary body holds the hard, biconvex,
transparent lens in place.
7. The iris is the colored part of the eye
surrounding the pupil. It regulates the amount
of light that can enter the pupil.
8. The interior of the eye is divided into two
fluid-filled compartments. The anterior
compartment is filled with aqueous humor;
and the posterior compartment is filled with
vitreous humor. These fluids help maintain
ocular pressure, bend light, and hold the
retina and lens in place.
9. There are more rods than cones in the retina.
These light-sensitive cells have two functions.
Rods are very sensitive to light and function in
dim light; cones produce color sensations and
require a lot of light.
10. The rods and cones synapse with the
bipolar sensory cells of the retina. These
cells synapse with the optic nerve, which
reaches the thalamus of the brain to
synapse with the visual cortex of the
occipital lobe of the cerebrum for
interpretation.
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The Sense of Hearing and Equilibrium
1. The external, middle, and inner ear contain the
organs of balance, or equilibrium, and hearing.
2. The visible, flexible, external ear is called the
auricle. It directs sound waves to the ear canal
called the external auditory meatus.
3. The ear canal is lined with hairs and
ceruminous glands that produce earwax to
protect the delicate eardrum, or tympanic
membrane, from foreign objects.
4. The middle ear contains the auditory ossicles:
the malleus or hammer, the incus or anvil, and
the stapes or stirrup. These bones transmit
sound vibrations from the tympanic
membrane, which vibrates due to sound
waves, to the oval window.
5. There are two openings on the medial side of
the middle ear: the oval window and the
round window, which connect the middle ear
to the inner ear.
6. The middle ear also contains the auditory or
eustachian tube, which connects to the
pharynx and allows for equalized air pressure
between the outside world and the middle ear,
thus not impairing hearing.
7. The inner ear consists of fluid-filled
interconnecting chambers and tunnels in the
temporal bone. It contains the cochlea
involved in hearing and the semicircular canals
and vestibule involved in balance.
REVIEW QUESTIONS
1. Name the four principal parts of the brain and
their subdivisions where appropriate.
2. Name the complex functions of the
hypothalamus.
3. Name the 12 cranial nerves; include their
Roman numeral designation and their
functions.
*4. Explain how the hypothalamus of the brain
and the autonomic nervous system allow us to
fight or flee in a stressful situation.
* Critical Thinking Question
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Fill in the Blank
Fill in the blank with the most appropriate term.
1. The brain is protected by the _______________
bones, the ________________, and
__________________ fluid.
2. Cerebrospinal fluid acts as a ________________
and circulates __________________.
3. Cerebrospinal fluid circulates in the
____________________ space and the four
____________________ of the brain.
4. Crossing of tracts in the medulla oblongata is
known as ____________________.
5. The ____________________ of the midbrain is
a reflex center that controls movement of the
head and eyeballs and head and trunk in
response to visual and auditory stimuli.
6. Folds on the surface of the cerebrum are
called ____________________ and intervening
grooves are called ____________________.
7. The two hemispheres of the cerebrum are
connected by a bridge of nerve fibers called
the ____________________.
8. The four main lobes of each cerebral
hemisphere are: ____________________,
____________________, __________________,
and __________________.
9. The cerebellum functions in coordinating
____________________ movements and
keeping the body ____________________.
10. The two subdivisions of the autonomic
nervous system are the ____________________
system, which stimulates and involves energy
expenditure, and the ____________________
system, which is mainly restorative.
Matching
Place the appropriate number in the blank provided.
____ Olfactory cortex 1. Transparent sclera
____ Taste cortex
2. Regulates light entering
____ Tears
eye
____ Cornea
3. Rods and cones
____ Choroid layer
4. Posterior compartment
____ Retina
of eye
____ Iris
5. Holds lens in place
____ Ciliary body
6. Visible portion of the
____ Pupil
external ear
____ Aqueous humor 7. Temporal and olfactory
____ Vitreous humor
lobes
____ Auricle
8. Hearing
____ Ceruminous
9. Balance
glands
10. Earwax
____ Cochlea
11. Parietal lobe
____ Semicircular
12. Anterior compartment
canals
of eye
13. Colored part of eye
14. Blood vessels and pigment cells
15. Lacrimal gland
16. Outermost layer of the
eye
17. Blind spot
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LABORATORY EXERCISE: THE NERVOUS SYSTEM
Materials needed: A model of a human
brain, a sheep or cow eye for dissection,
a model of the external and internal ear, a dissecting pan, and a dissecting kit.
1. Obtain a model of a preserved human brain
showing a frontal and cross section. These are
available from a biologic supply company and
will be provided by your instructor. Identify
the various parts of the brain referring to
Figure 11-1 in your text. In addition identify
the four ventricles of the brain.
2. Obtain a sheep or cow eye from your
instructor. Make a transverse cut through the
eye with your scalpel. Refer to Figure 11-10
in your text. Identify the three layers of the
eye: the hard white outer sclera; the black
choroid in the middle; and the innermost
retina. Locate the biconvex lens. Anterior to
the lens is the aqueous humor and posterior
to the lens is the vitreous humor. Note the
dark delicate iris surrounding the opening
into the lens, the pupil. If you look carefully
at the rear half of the eye, you will see a
shiny greenish blue material. This is the
tapetum. There is no tapetum in a human
eye but in a cow or sheep eye this area
reflects light causing the animal’s eye to
glow in the dark when light is shined on it.
3. Obtain an anatomic model of the ear from
your instructor. Identify the auricle and
external auditory meatus of the outer ear, the
middle ear, and the structures of the inner
ear. Refer to Figure 11-12 in your text.